1. Field
The present invention relates generally to the field of wireless communication systems, and more specifically to high data rate (xe2x80x9cHDRxe2x80x9d) data transmission in wireless communication systems.
2. Background
In wireless communication systems several users may share a common communication channel. To avoid conflicts arising from several users transmitting information over the communication channel at the same time, some allocation of the available channel capacity to the users is required. Allocation of user access to the communication channel is achieved by various forms of multiple access protocols. One form of protocol is code division multiple access (xe2x80x9cCDMAxe2x80x9d) and another form of protocol is time division multiple access (xe2x80x9cTDMAxe2x80x9d).
In CDMA systems each user uniquely encodes its communication signal into a transmission signal in order to separate its signal from those of other users. The encoding of the message signal spreads its spectrum so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the message signal. For this reason CDMA systems are also referred to as xe2x80x9cspread spectrumxe2x80x9d systems. In TDMA systems each user transmits its communication signal in a uniquely assigned time slot. The time slots do not overlap so that each user""s signal is separated from those of other users.
HDR data transmission is a technology that can provide data transmission in a standard CDMA voice communication channel. HDR can be used to enhance data capabilities in existing CDMA networks or in stand-alone data networks. For example, HDR can provide data transmission rates of approximately 2.4 million bits per second (xe2x80x9cMbpsxe2x80x9d). With existing CDMA networks, some number of channels are changed from voice to data. HDR uses a combination of CDMA and TDMA to share each communication channel among several users. However, HDR assigns time slots on an as-needed basis rather than on a fixed basis as with TDMA.
FIG. 1 illustrates an example of communication channels used for transmitting data using HDR in a CDMA wireless communication system. Communication system 100 shown in FIG. 1 might be, for example, part of a cdma2000 spread spectrum communication system. As shown in FIG. 1, mobile unit 102, which can be an HDR modem, communicates with base station 112 over a communication channel provided by radio frequency signal propagation between mobile unit antenna 110 connected to mobile unit 102 and base station antenna 114 connected to base station 112. Mobile unit 102 may optionally be connected to a computer, such as a personal computer (xe2x80x9cPCxe2x80x9d), for example, PC 104. PC 104 can be connected to mobile unit 102 by data link 106, which can be a serial cable connected to an RS-232 port, for example. (RS-232 refers to Recommended Standard-232, a standard for serial transmission between computers and peripheral devices, now officially referred to as TIA/EIA-232-E.)
The communication channel includes forward data channel 116, which can be used for carrying user data, indicated in FIG. 1 by an arrow which points in the forward direction from base station 112 to mobile unit 102. The communication channel also includes forward control channel 118, which can be used for carrying signaling information and power control information, indicated in FIG. 1 by an arrow which also points in the forward direction. The communication channel further includes reverse data channel 120, which can be used for carrying user data, indicated in FIG. 1 by an arrow which points in the reverse direction from mobile unit 102 to base station 112. The communication channel also includes reverse control channel 122, which can be used for carrying signaling information and power control information, indicated in FIG. 1 by an arrow which also points in the reverse direction.
HDR data rates can vary depending on certain factors. For example, HDR data rates can vary depending on the distance from the mobile unit, i.e. the HDR modem, to the base station. HDR data rates can also vary from time slot to time slot, for example, depending on the instantaneous signal quality, generally measured as signal to noise ratio, of the communication channel. As seen in FIG. 1, the communication channel also includes data request channel (xe2x80x9cDRCxe2x80x9d) 124. DRC 124 is used to specify either the maximum data rate that the instantaneous signal quality of the communication channel can support or the null data rate as specified by Interim Standard 856 (xe2x80x9cIS-856xe2x80x9d), the technical specifications for the HDR air interface.
When a high data rate modem such as HDR modem 102 is coupled to PC 104 through data link 106, for example, an RS-232 port, which has a lower data rate, a xe2x80x9cbottleneckxe2x80x9d problem arises. In the HDR modem, with data coming into the data buffer at the HDR data rate of approximately 2.4 Mbps and leaving the data buffer at the RS-232 data rate of approximately 115 thousand bits per second (xe2x80x9cKbpsxe2x80x9d), it is possible for data to xe2x80x9coverflowxe2x80x9d the buffer, i.e. data is lost. The bottleneck problem can be partially solved by providing a larger data buffer, but at differing data rates it is possible for data to overflow the buffer and be lost regardless of the data buffer size. Another problem which arises is that occasionally in wireless systems, data needs to be retransmitted, due, for example, to the varying signal quality of the communication channel which can be caused by noise or interference. In general, the retransmit data is given a higher priority for transmission than other data, for example, in order to maintain orderly filling and emptying of data buffers.
Various protocols exist for controlling the data rate on a data link, also referred to as xe2x80x9cflow control.xe2x80x9d As an illustration, flow control can be provided between a modem and a PC in hardware, for example, an RS-232 data link, by providing a separate control link, one for the modem and one for the PC, so that each can start and stop data flow from the other. Thus, if the data buffer in the modem starts to fill up, the modem can stop data flow from the PC until the modem can xe2x80x9ccatch upxe2x80x9d by processing data out of its buffer. Flow control can also be provided between a modem and a PC in software, such as an Xon/Xoff protocol, by including special control characters, i.e. characters which cannot be interpreted as data, in the data stream. The special control characters can be used by the modem and the PC so that each can start and stop data flow from the other. Thus, if the data buffer in the modem starts to fill up, the modem can stop data flow from the PC by sending a special character to stop data flow, until the modem can xe2x80x9ccatch upxe2x80x9d by processing data out of its buffer, and then sending another special character to restart data flow.
Neither the hardware flow control nor the software flow control protocols are sensitive to the requirements for re-transmitting data in wireless systems, in general, or the requirements for transmission of retransmit and other priority data in HDR technology, in particular. Thus, even though HDR technology provides separate data and control channels and a separate data request channel (xe2x80x9cDRCxe2x80x9d), application of existing flow control protocols in HDR typically creates problems with transmission of retransmit and other priority data. For example, existing flow control protocols can stop data transmission for too long so that retransmit or priority data is either lost or needs to be retransmitted unnecessarily. According to the IS-856 technical standard for HDR modems, the DRC channel is used to specify only the null data rate or the maximum data rate that the instantaneous signal quality of the communication channel can support. The DRC channel, thus, cannot be used to specify any optional data rates, such as a data rate compatible with a particular modem buffer and modem processing speed.
Thus, there is a need in the art for flow control in HDR data transmission in wireless communication systems. Moreover, there is a need in the art for adjusting the data rate of HDR data links in wireless communication systems without interfering with transmission of retransmit and other priority data.
Embodiments disclosed herein address the above stated needs by providing flow control in HDR data transmission in wireless communication systems. Moreover, an embodiment adjusts the data rate of HDR data links in wireless communication systems without interfering with transmission of retransmit and other priority data.
In one aspect of the invention, communication between a mobile unit and a base station is performed at a data transfer rate selected from a finite set of HDR data transfer rates. Meanwhile, the mobile unit continually computes a moving average of the data transfer rates. For example, the mobile unit computes the moving average of the data transfer rates over a pre-determined number of HDR time slots. The mobile unit also ascertains a supportable data rate for receiving data. For example, the supportable data rate is ascertained by measuring the signal quality of the communications channel being received from the base station. The supportable data rate can be selected from a finite set of HDR data transfer rates.
In order to maintain the moving average of the data transfer rate within a specified range, the mobile unit transmits maximum data rate requests or null data requests to the base station. The maximum data rates requested by the mobile unit are equal to or lower than the supportable data rate.